The easiest way I can think of is to open the image header file. You can find it under ...\netbackup\db\images\<client_name>\1######### \ folder. The header files are the ones without any extension. You can open it with any text editor.
Look for this entry in the header file:
CLIENT_TYPE
The number following CLIENT_TYPE indicates the policy type.
The full list can be found in NBU Commands manual.
0 = Standard
4 = Oracle
6 = Informix-On-BAR
7 = Sybase
8 = MS-SharePoint
10 = NetWare
13 = MS-Windows
14 = OS/2
15 = MS-SQL-Server
16 = MS-Exchange-Server
Hope this helps!